1. Field of the Invention
The subject invention pertains to reactive compositions useful for preparation of microcellular poly(urethane-urea) parts by reaction injection molding (RIM). More particularly, the subject invention pertains to a RIM system wherein a major portion of the polyisocyanate component is an isocyanate terminated prepolymer prepared by the reaction of a polyoxypropylene glycol with a methylenebis(phenylisocyanate).
2. Description of the Related Art
Reaction injection molding (RIM) and its closely related variations reinforced RIM (RRIM) and structural RIM (SRIM) are by now widely used to prepare complex parts, particularly in the transportation industries. A common use of this technology is to produce fascias for automobiles, for example.
The varieties of RIM, which for purposes of this application includes within the term "RIM" RRIM and SRIM, are made practical by the use of reactive resin systems which have high flow rates but short cure rates, or gel times. These two requirements are necessary, the former to enable complete filing of an often large and complex mold; the latter to enable a high rate of productivity from the mold. In order to achieve these somewhat conflicting goals, resin systems have been developed which rely on the use of complex mixtures of components.
In U.S. Pat. No. 4,218,543, herein incorporated by reference, is disclosed the use of sterically hindered aromatic diamines such as 1-methyl-3,5-diethyl-2,4-diaminobenzene,1-methyl-3,5-diethyl-2,6-diaminobe nzene, and other diamines in which the reactivity of the diamine is lessened by the presence of alkyl groups ortho the amino group, these diamines hereinafter referred to as "hindered aromatic diamines". The isocyanates useful were said to be 4,4'-methylenebis(phenylisocyanate) (4,4'-MDI) or low molecular weight adducts prepared by joining two molecules of 4,4'-MDI by means of a urethane or carbodiimide linkage. The RIM systems of the '543 patent exhibit low gel times, on the order of 3 seconds or so, but have less than ideal flowability.
U.S. Pat. No. 4,374,210 discloses an improvement in poly(urethane-urea) RIM systems by use of an isocyanate terminated prepolymer derived from reacting 4,4'-MDI with a polyoxyethylenepolyoxypropylene copolymer diol or triol, a polyoxytetramethylene glycol, or a polyester diol or triol. Such prepolymers are said to increase utilization of the RIM machinery due to the ability to operate at nearly a 1:1 ratio of the A and B sides. However, the systems of U.S. Pat. No. 4,374,210 lack optimal flow and RIM parts prepared according to this method suffer from increased water absorption.